The Earth’s movements affect the daily life of all living beings that inhabit it. These movements have always been reasons for discussion and debate among human beings, having influenced the scientific thinking of every civilization that existed.
Great scientists and astronomers such as Nicholas Copernicus, Fiolao of Crotona, Hipparchus of Nicaea, James Bradly Johannes Kepler and Isaac Newton were interested during their research into the Earth’s motions, including translation.
Among the most important features of the translation movement are:
– The orbit described by the Earth is elliptical and with the Sun in one of the focuses, as determined by Kepler’s laws of planetary motion. An observer at the North Pole would say that it does this counterclockwise (levorotatory).
– The total length of the elliptical orbit is about 930 million kilometers.
– The eccentricity of this ellipse is so small (calculated at 0.017), that the Earth’s orbit can approach very well as a circumference whose approximate radius is 150 x 10 6 km. If the orbit is accurately drawn, it cannot be visually distinguished from a circle. In fact, the lower half axis of the orbit is approximately 99.98% of the length of the major half axis.
– The Earth follows this path at a speed of about 30 km / s in a plane called the ecliptic , whose perpendicular when passing through the center of the Earth defines the poles of the ecliptic . The Earth’s axis of rotation is tilted relative to this line by 23.5°, exposing the northern hemisphere more to the sun’s rays during the summer months and vice versa during the winter.
The cause of the Earth describing an elliptical orbit around the king star is the gravitational pull it exerts on it and the nature of this force, which depends on the inverse square of the distance 1 / r 2 .
In the late 16th century, German astronomer Johannes Kepler (1571-1630) discovered that the actual trajectories of planets around the Sun were elliptical. And this fact later provided Isaac Newton with the basis for establishing the law of universal gravitation.
An ellipse is the geometric location of points at which the sum of the distances for two points called foci is constant. In Earth’s orbit, the Sun is at one of the focuses.
The flatter an ellipse, the more different the semi-major axis and the semi-minor axis. The eccentricity of the ellipse is the parameter that measures this characteristic. If it’s 0, which is the smallest possible value, it’s a circle.
Even with a small eccentricity, the Earth passes during the month of January through a point closer to the Sun, called perihelion , 147.1 million kilometers from the Sun. And aphelion is the most distant, occurring in July and measuring 152. 6 million km.
The period of movement of the Earth’s movement
Kepler’s laws for planetary motion were empirically established from numerous measurements. They establish that:
– Planetary orbits are elliptical
– The area swept by the vector’s radius during a given time interval is the same throughout the movement.
– The square of the period ( T 2 ) is proportional to the cube of the average distance between the planet and the sun ( R 3 ), where C is the proportionality constant, the same for any planet:
T 2 = CR 3
The value of C can be calculated using already known data for Earth and its units in the International System are s 2 / m 3 .
Earth’s movements are closely linked to the measurement of weather and seasonal changes in climate, in which temperature and hours of light and darkness vary. Both factors and their periodicity resulted in human activities being governed by times set in calendars.
The translation movement defines the length of the year , during which seasons occur and the stars in the sky change. During the summer, those that are visible at night, “going out” in the east and “putting in” in the west in the morning, do the opposite during the winter.
Furthermore, the climate changes according to the time the earth’s surface is exposed to sunlight. Seasons are a combined effect of the earth’s translational motion and the tilt of the axis of rotation relative to the orbital plane.
The Earth completes a circle around the Sun in 365 days, 5 hours, 48 minutes and 45.6 seconds. This assumes that it is taken as a reference to the Sun, which will be considered fixed.
This is the definition of “solar year” or “tropical year”, the time between two consecutive spring equinoxes. The equinoxes are times of the year when day and night have the same duration anywhere on the planet. They take place on March 22nd and September 22nd.
As this time exceeds 365 days, but you need to keep solstices and equinoxes on the same days of the year and that this has an integer number of days, the leap year concept is introduced.
About 6 hours are added each year, so that after 4 years they accumulate 24 hours or a whole day: a year of 366 days or a leap year. The extra day is granted for the month of February.
On the other hand, the “astronomical year” is measured according to the time it takes for the Earth to pass successively twice through the same point. But this year is not the only one that sets the calendar.
Ground stations and zonal divisions
The movement of the terrestrial translation, plus the inclination of the axis of rotation in relation to the ecliptic poles (elliptical obliquity), causes the planet to move away from or closer to the sun and vary the exposure to the sun’s rays, giving rise to the exposure to the sunlight for the seasons: the equinoxes and solstices.
The intensity and duration of seasonal changes vary by location on Earth. In this way, the following zonal divisions are defined:
– the equator
– the tropics
– The temperate zone
– the polar circles.
– The poles
At the equator, the sun’s rays have maximum verticality and the days and nights have the same duration throughout the year. At these points, climatic variations depend on the height above sea level.
As it moves towards the poles, the incidence of sunlight is increasingly oblique, resulting in changes in temperature and an inequality between the length of days and nights.
The solstices are two times of year that occur when the Sun reaches its height more or less apparent in the sky, and the length of day and night are the highest of the year (summer and winter solstice, respectively).
In the Northern Hemisphere, summer is from June 20th to 23rd and winter is from December 21st to 22nd. In the first case, the sun reaches its maximum height at midday on the imaginary line known as the Tropic of Cancer (longest day of the year) and in the second its height is minimal .
The dates present small variations due to another terrestrial movement: that of precession .
During this period, the sun’s rays have a greater impact on the northern hemisphere (summer) and, conversely, on the southern hemisphere (winter). On the other hand, the Sun is always visible at the north pole, while the south pole is not illuminated, as can be seen in the figure.
For the southern hemisphere, the situation is reversed: between 20 and 21 December, the sun is at its point more high noon at the Tropic of Capricorn, and the summer solstice which gives rise to the hot season. And June 20-21 is the minimum and is the winter solstice (longest night of the year).
During the winter solstice, the North Pole remains dark, while the South Pole is summer and daylight is permanent.
During the equinoxes, the Sun reaches its zenith or highest point perpendicular to the equator, so that solar radiation reaches the same inclination in both hemispheres.
The times when this occurs are March 21st and 22nd: spring equinox in the northern hemisphere and autumn in the southern hemisphere and September 22 and 23 in reverse: autumn in the north and spring in the south.
During the equinoxes, the sun rises in the east and sets in the west. The figure shows that illumination is evenly distributed in both hemispheres.
The duration of the four seasons is approximately the same in days, averaging around 90 days with slight variations.